1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus.
2. Description of the Related Art
As a method of separating and conveying stacked sheets, such as documents and recording sheets, a separating and conveying method using frictional force and a separating and conveying method based on air suction have been used. The separating and conveying method using frictional force typically uses, for example, a rubber feeding roller, and as a result the frictional force changes over time due to abrasion and other factors, such that feeding performance deteriorates. Further, when sheets non-uniform (i.e., varying) coefficient of friction or sheets having different coefficients of friction are separated and conveyed in the same separating and conveying operation, a feeding failure occurs in some cases, which includes simultaneous multiple feeding of a plurality of sheets and a failure to separate sheets. Further, in some cases, the sheets are stained due to a configuration that separates the sheets by applying pressure thereto in the sheet feeding operation.
Meanwhile, the separating and conveying method using air suction is a non-frictional separation method not relying on the coefficient of friction of rollers and sheets. The method, however, uses an air suction blower and an air duct. Thus, the sheet conveying device is increased in size, and air suction sound itself is noise. Therefore, the device is not suitable for use in an office environment.
In view of the above, an electrostatic method as one type of non-frictional separation method has been proposed, which generates an electric field in a dielectric belt and brings the dielectric belt into contact with a sheet to simultaneously attract the sheet and separate the sheet from other sheets. According to the electrostatic method, an attraction belt that serves as a dielectric belt wound around a plurality of rollers is supplied with an alternating charge, and is brought into contact with a surface of the uppermost sheet of a sheet stack to attract the uppermost sheet of the sheet stack. The attraction belt having attracted the uppermost sheet is then moved in a direction separating the uppermost sheet from the sheet stack, thus separating the uppermost sheet of the sheet stack from the sheet stack. Then, the attraction belt is driven to rotate and convey the uppermost sheet attracted thereto to a conveying roller pair. The electrostatic method as described above is advantageous in preventing, for example, abrasion, damage to a sheet, and noise, and reducing the size of the device.
If the uppermost sheet of the sheet stack includes an undulating or curled portion, however, such a portion sometimes results in an insufficient area of contact between the uppermost sheet of the sheet stack and the attraction belt and therefore insufficient electrostatic. attraction of the uppermost sheet to the attraction belt. If the attraction belt is moved in the direction separating the uppermost sheet from the sheet stack in this state of insufficient electrostatic attraction, the uppermost sheet is separated from the attraction belt instead and can cause conveyance failure.
The present inventors studied the insufficient electrostatic attraction of the uppermost sheet of the sheet stack to the attraction belt caused by an undulating or curled portion of the sheet, and found the following. That is, when the attraction belt is driven to rotate in a typical configuration, the attraction belt is stretched by a plurality of tension rollers with relatively high tension to favorably drive the attraction belt to rotate without causing slippage between the attraction belt and one of the tension rollers that serve as a drive roller. If the tightly stretched attraction belt is brought into contact with the uppermost sheet of the sheet stack, therefore, the attraction belt fails to fit the shape of the undulating or curled portion of the uppermost sheet, and consequently results in an insufficient area of contact between the uppermost sheet and the attraction belt.